1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus such as a printer, a copier or a facsimile for heating and pressing an unfixed toner image or an unfixed ink image carried on a recording medium, to fix the image on the recording medium.
2. Description of the Related Art
In the image forming apparatus for forming a visible image with a sticking toner, a fixing device widely used for fixing the toner image on the recording medium includes a heating roller and a pressure roller. In this fixing device, the heating roller and the pressure roller are pressed to make contact each other so that the unfixed toner image is heated/pressed onto the recording medium by interposing the recording medium carrying the toner image into their contacting portion (or a nip portion). The heating roller to be mostly used in such fixing device includes a hollow core of aluminum and a heater disposed in the core, and is coated on its surface with a fluorine-containing resin or the like having an excellent releasing property.
When the recording medium carrying the unfixed toner passes through the aforementioned nip portion of the fixing device, a phenomenon that a portion of the unfixed toner on the recording medium may stick to a surface of the heating roller, that is, the offset may occur. If this toner offset occurs, the image may partially miss, or the offset toner may stick to the coming next recording medium thereby to blot the image.
Besides, the cause for the toner to offset on the heating roller has been newly found to come from the following mechanism.
A kind of the cut paper to be used as the recording medium contains calcium carbonate as the filler. When the recording medium containing the calcium carbonate is clamped and transferred by the nip portion, at which the heating roller and the pressure roller abut against each other, the calcium carbonate sticks as the paper dust to the surface layer of the heating roller. This calcium carbonate is liable to be charged in a positive polarity so that the offset of the toner is induced if the calcium carbonate sticks to the surface of the heating roller.
In most of the related arts, moreover, the heating roller is set to have a potential of the same negative polarity as that of the toner so that an electric repulsive force is generated between the heating roller and the toner charged to the negative polarity thereby to prevent the offset of the toner. If, therefore, the paper containing the calcium carbonate is used as the recording medium, the paper dust of the calcium carbonate or the like charged in the positive polarity is so electrostatically attracted to the surface layer of the heating roller set to have the potential of the negative polarity that it easily sticks to the surface layer. The paper dust of the calcium carbonate or the like having stuck to the surface of the heating roller raises the surface potential of the heating roller so that the toner charged to the negative polarity is electrostatically attracted by the heating roller thereby to cause its offset.
At the nip portion where the heating roller and the pressure roller are pressed to make contact each other, on the other hand, the back of the recording medium, i.e., the surface opposed to the surface on which the toner image is carried, is directly pressed onto the peripheral surface of the pressure roller. If the recording medium is the paper containing the calcium carbonate, much paper dust of the calcium carbonate or the like having the charge of the positive polarity sticks to the back of the recording medium so that it transfers to the surface of the pressure roller when it passes through the nip portion. The paper dust having stuck to the pressure roller transfers to the heating roller abutting against the pressure roller. The paper dust of the calcium carbonate or the like raises, if much having stuck to the surface of the heating roller, the surface potential so that the unfixed toner charged in the negative polarity on the recording medium is electrostatically attracted and transferred to the heating roller thereby to cause the offset of the toner.
Although the device for fixing the toner image has been described on its problems, the printer using ink, the ink jet type image forming apparatus and the like may use a device for fixing the ink image early by inserting the recording medium between the circumferentially moving heating roller and the pressure roller to be pressed to make contact with the former, and by heating and pressing the recording medium so that the ink may be earlier fixed. This device is encountered by the offset of the ink image and by the blotting of the image, if much dust of the calcium carbonate sticks to the peripheral surface of the heating roller, which makes direct contact with the surface of the recording medium carrying the ink image.